1. Field of the Invention
The present invention relates to a control system for an internal combustion engine particularly used in an automobile.
2. Description of the Related Art
An automotive engine is desired to satisfy the demands of reducing the fuel consumption rate and improving the acceleration.
As a method for satisfying these demands, there has been known in the art a system for supplying the internal combustion engine with a lean air/fuel mixture, as disclosed in Japanese Patent Laid-Open No. 59-224499 (1984). However, this method is accompanied by a first problem that a sufficient acceleration cannot be attained because of the leanness of the mixture.
In order to eliminate this shortage of acceleration, on the other hand, it is effective to prepare a rich mixture and supply it to the engine during acceleration. However, there arises a second problem that the engine torque will suddenly change to deteriorate the drivability when the mixture makes a transition from a rich one to a lean one or vice versa.
In order to eliminate this sudden change of the torque, it is conceivable to have a gradual transition of the richness of the mixture. However, there exists between the lean mixture zone and the rich mixture zone the region, i.e. about 15 to 18 in air/fuel ratio, where nitrogen oxides (NO.sub.X) are generated at the maximum to adversely affect the reduction of the noxious components in engine exhaust gas.